The invention relates to inflatable bags for use in forming a false bottom in a storage tank.
Cylindrical storage tanks of the type used for the bulk storage of liquids, e.g. fuels, oil and chemicals, have traditionally been designed with the main drain outlet located in the tank wall a few feet above the tank floor. This ensures that in normal use the tank is never fully emptied, although there will also be a sump drain whereby the remaining liquid and any accumulated sludge can be removed. For many purposes, i.e. when storing certain liquids, this arrangement is preferred. However, at other times, e.g. for other liquids, it is preferred to be able to extract virtually all the stored liquid through the main outlet, i.e. so that no liquid remains when the tank has been emptied to the level of this main outlet, an advantage of this method of operation being a reduction in the inventory of stored liquid on hand.
The purpose of the present invention is to provide means for forming a removable false bottom in such a tank, thus providing it with the versatility to operate in either of the above-described modes, i.e. in the manner in which it was built with the floor a few feet below the main outlet level, or with an effective floor (false bottom) substantially at such level. A basic requirement of the invention is to provide means for forming such a false bottom in a manner that enables it to be relatively easily set up or dismantled without involving major engineering work.
Storage tanks are generally large structures, e.g. 120 feet in diameter, and of substantial height. They often have floating roofs that slide up and down as the volume of liquid in the tank varies, and they are thus effectively closed. Any attempt to use a metal false bottom would involve a massive structure and, when there is a roof, its removal before the bottom could be installed. Access to the inside of such a tank is generally provided by a manhole, usually located in the side at ground level, and hence, a further important factor is that the means for forming the false bottom should be capable of being taken into the tank or removed therefrom through such a manhole.
A solution to these problems is to use liquid tight, inflatable bags to form the false bottom. For example, the false bottom can be constructed of a number of inflatable bags that can be individually folded when deflated (in order to be able to pass through the manhole), but, once in place on the floor of the tank, can be inflated with a suitable liquid, e.g. water or brine, heavier than the liquid to be stored, e.g. oil. It is possible to arrange the shapes of the bags so that they occupy the entire surface of the tank floor, while extending up to approximately the level of the main outlet, so that the desired false bottom effect can be achieved. Alternatively, if the floor of the tank is cluttered with filling pipes, supports for such pipes, sump equipment and/or guides or supports for the floating roof, the number and arrangement of the bags can be varied to accommodate this equipment. If the bags are of the correct shape and are packed closely together, once they have been inflated their adjacent sides will press firmly against each other and against the cylindrical wall of the tank, thus making a virtually liquid tight bottom. There may be some minor seepage of the stored liquid between adjacent bags or between the bags and the tank wall, but this will be no large amount and will represent no serious disadvantage.
It is of course important that the bags themselves be reliably liquid tight in order to avoid the water or brine with which they are filled contaminating the stored liquid. Brine will be preferred when the stored liquid is oil, since, if there should be a leaky bag, the brine will be more immiscible with the oil than plain water would be.
It is also important that the material of the bags be chemically inert to the stored liquid. A convenient material to use will be a coated fabric, e.g. a metalised fabric, or one coated with a PVC alloy that withstands attack by oil. The material of the bags may require modification for different stored liquids in order to retain this necessary chemically inert relationship.